Much interest has arisen during recent years in the development of the last-mentioned apparatus for the infusion of drugs in a relatively continuous manner compared to the conventional intermittent form of administration by syringe injection, such infusion affording closer treatment control, allowing a reduction of overall drug dosage in many cases, and providing other benefits. Typically, the powered infusion apparatus in question has involved an incremental drive by a battery-powered motor, through a nut-and-leadscrew mechanism, to push the free end of the plunger of a syringe coupled to the patient by way of a cannula. Examples of such motor driven apparatus are described in UK Pat. No. 1,528,385 and these examples have been used successfully with standard syringes to provide individual discharge periods of up to 48 hours.
It is evident that the benefits of apparatus such as that just described would be improved by extension of the discharge period for an individual syringe, up to the order of a week or more, say, but this is problematical. Difficulty arises particularly when initiating operation after loading a syringe in the apparatus insofar as infusion does not commence until backlash in the drive system has been taken up and such movements represent a significant drive period. While the backlash can be virtually eliminated in much of the drive system, the nut-and-leadscrew mechanism normally involves the use of a releasable half nut which allows free leadscrew movement to facilitate syringe loading, and the subsequent re-engagement of the nut introduces a variable amount of backlash between the leadscrew and syringe plunger.